1. Technical Field
The present invention relates to rotors for rotating electric machines that are used in, for example, motor vehicles as electric motors and electric generators.
2. Description of Related Art
There are known rotors for rotating electric machines that are used in, for example, motor vehicles as electric motors and electric generators. Those rotors include a rotor core and a plurality of permanent magnets. The rotor core is configured to be disposed in radial opposition to a stator of the rotating electric machine. The rotor core has a plurality of pairs of magnet-receiving holes formed therein. Each pair of the magnet-receiving holes is arranged in a substantially V-shape that opens toward the stator side. Each of the permanent magnets is received in a corresponding one of the magnet-receiving holes of the rotor core. Further, for each pair of the magnet-receiving holes, the two corresponding permanent magnets which are respectively received in the two magnet-receiving holes of the pair are arranged so as to together form one magnetic pole of the rotor. In addition, for each pair of the magnet-receiving holes, there is formed a corresponding center bridge that extends in a radial direction of the rotor core between the two magnet-receiving holes of the pair to separate them from each other.
Japanese Unexamined Patent Application Publication No. 2011-211860 discloses a technique of providing, for each pair of the magnet-receiving holes of the rotor core, a pair of magnetic flux barriers (or magnetic voids) that respectively extend from magnetic pole centerline-side ends of the pair of the magnet-receiving holes toward a longitudinal axis of the rotor core (or radially inward). Further, the corresponding center bridge is radially extended toward the longitudinal axis of the rotor core so as to also separate the pair of the magnetic flux barriers from each other. Furthermore, the corresponding center bridge has an internal barrier formed at its circumferential center and is thus circumferentially divided by the internal barrier into two parts.
Consequently, by providing the magnetic flux barriers for each pair of the magnet-receiving holes and the internal barrier in the corresponding center bridge, it becomes difficult for magnetic flux to flow along the d-axis (or along the centerline of the magnetic pole). In other words, the d-axis inductance is reduced. As a result, it becomes possible to reduce magnetic flux leakage along the d-axis toward the longitudinal axis of the rotor core (or radially inward). Moreover, by providing the magnetic flux barriers and the internal barrier, the weight and thus the inertia of the rotor core can be reduced.
However, with the technique disclosed in the above patent document, the radial length of the corresponding center bridge is increased by the radial length of the magnetic flux barriers. Consequently, the circumferential width of the corresponding center bridge becomes small relative to the radial length, thereby making it difficult to secure a sufficient strength of the corresponding center bridge to withstand the centrifugal force during rotation of the rotor. Moreover, since the corresponding center bridge is circumferentially divided by the internal barrier into the two parts, the circumferential width of each of the two parts of the corresponding center bridge is approximately half the circumferential width of a conventional center bridge which has no internal barrier formed therein. Consequently, the strength of the corresponding center bridge is considerably lowered in comparison with the strength of the conventional center bridge.